Small-angle neutron scattering technique was used to study the effects of the microstructural change on the miscibility and the effective segmental interaction parameter (chi) of deuterated polysulfone (d-PSU) and protonated PSU (PSU) blends. For the studies of the end- group effects on the miscibility and the chi parameter, we prepared the three kinds of blends of d-PSU with PSU terminated with mostly Cl end group (PSU-Cl), PSU terminated with mostly OH end-group (PSU-OH) and PSU terminated with phthalic anhydride group (PSU-PhAh). For the studies of copolymerization effects, we prepared a copolymer of a sulfone unit and a monomer unit with COOH group (DPA unit) (PSU-COOH). The chi parameters were estimated from the SANS profiles as a function of temperature for the blends of d-PSU with various modified PSUs and summarized in a figure in the text. Further, in the blends of d-PSU and PSU-PhAh, the chi parameter was found to be very sensitive to the molecular weight of PSU-PhAh: if the molecular weight of PSU-PhAh is small, the blend was shown to be phase-separated, i.e. the effect of the end-group of PhAh on the miscibility and the chi parameter was found to be very large. In the blend of d-PSU and PSU-COOH, the phase behavior was found to strongly depend upon the content of DPA unit included in PSU-COOH. The larger the content of DPA unit, the larger the chi parameter. If the content was larger than a critical value, the blend was found to phase-separate.